JPS6125095B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention eliminates trace amounts contained in an oxygen-containing gaseous mixture by applying an ion-repelling electric field directed in the direction opposite to the gas flow to the ions generated in a mixed gas sample under flow conditions. The present invention relates to an apparatus for detecting chemical species in vapor form. "Species" herein refers to vapor molecules of substances that are capable of forming relatively stable ions in the presence of oxygen under the influence of an applied electric field; It should have a lower mobility due to the movement of other ions that may be generated in the gaseous mixture it contains. Such species include certain substituted hydrocarbons having at least one strongly electronegative or electronegative atom or group of atoms and heavy atomic weight halogen atoms such as bromine and iodine in the molecule. The device of the invention can be used to measure the presence of solids, liquids or mixtures thereof which release the above-mentioned species in vapor form at normal temperature and pressure. Examples of such substances are explosives such as dynamite, toxic vapors such as narcotics and pesticides. The device of the invention can also be used to detect leaks into the atmosphere of gases from pipelines, chemical plants, etc. containing chemical species, e.g. tracer substances such as high atomic weight halogen atoms. can. When a sample of atmospheric gas containing trace amounts of chemical species of the above-mentioned type is ionized by an applied electric field, for example in the form of a corona discharge, ions are formed from the molecules of the chemical species together with ions of other components of the atmosphere. It has been known. Typically, the ions present have different ion mobilities. It is also possible to measure the mobility of various ionic species generated during corona discharge using ion-repelling electric field techniques, e.g. in Journal of Applied Physics, Vol. 47.
Vol. 6, No. 6, 1976. This paper includes the inverse mobility spectra of ion species in continuous gas flow conditions.
The method for recording the mobility spectrum is described. The spectra obtained are from oxygen and nitrogen and from an air atmosphere. No explanation is provided regarding the gas composition used other than identifying unspecified impurities that may be present in the gas mixture. It is an object of the present invention to obtain a device capable of detecting the presence of a chemical species of the above-mentioned type in an oxygen-containing gaseous mixture by applying an ion-repelling electric field of the above-mentioned species in continuous gas flow conditions. There is something to do. The devices and methods of the present invention take advantage of the relatively low mobility of ions formed from such species compared to the mobility of ions formed from other components of the gaseous mixture. In the present invention, by applying an ion repelling electric field directed in the direction opposite to the gas flow to the ions generated in the mixed gas sample under flow conditions, trace amount contained in the oxygen-containing gaseous mixture can be reduced. an apparatus for detecting a chemical species in vapor form, comprising means for passing said sample through a hollow body, said hollow body having in series an inlet, a first internal section, a second internal section, a third internal section;
an interior zone and an outlet, the first zone having means for ionizing at least a portion of the molecules of the sample containing molecules of said species, and ions of a certain polarity selected for migration with said sample to a second zone; means for promoting the flow of the sample through the second zone as a jet with a substantially uniform flow velocity; means for generating an ion repelling electric field in the opposite direction in the second zone; means for collecting polar ions, and during operation selected polar ions having an ion mobility greater than a certain value depending on the strength of the ion repelling electric field and the flow rate of the gas flow into the third zone. To provide a detection device characterized in that it is arranged in such a way that it can be prevented from occurring. In some configurations, the device is light in weight, portable, and can be operated without the use of a bottled gas supply. This device is relatively simple in construction and can be manufactured economically. This device is particularly advantageous when applied to the detection of trace species emitted from explosive materials in that only a short pre-run time is required to detect the species. The hollow body is comprised of a first tube and a second tube of conductive material joined in a butt-to-end relationship by a ring of insulating material, with the first section disposed within the first tube and the second and third sections disposed within the first tube. Can be installed in two pipes. The first tube contains the sample at the opposite end from the second tube.
an end wall having an inlet hole for introducing the sample into the section, and at the end adjacent to the second tube a disk of electrically conductive material, which disk is placed transversely to the gas flow; At least one hole may be provided in the disc at the end adjacent the second tube for introducing the sample into the second zone. The shape is such that it can pass through the second area. The second zone and the third zone can be partially separated by a disk of conductive material placed at a point remote from the ends of the second tube, the disk introducing the sample into the third zone. at least one hole leading from the second section to the third section for electrically contacting the disk with the tube wall; The means for passing the sample through the hollow body can be an electrically driven fan, which can be placed approximately at the outlet of the hollow body. The means for ionizing at least a portion of the sample molecules, including molecules of the species, may be an electrode located in the first zone or a radioactive ionization source located in the first zone. If a radioactive ionization source is used, means are provided in the first zone for selecting ions of a certain polarity from the population of ambipolar ions produced by the radioactivity. The means for selecting ions of a certain polarity that migrate with the sample to the second zone can be an electric field generated by a high DC voltage applied to the electrodes of the first zone. If the ionizing means is an electrode that produces a corona discharge during operation, it is advantageous to limit the discharge current to less than 100 nA;
It was confirmed that it is advantageous to limit the current to 30 nA or less. Current limitation can be achieved by inserting a resistor between the high voltage source and the discharge electrode. If a radioactive ionization source is used, the disk between the first and second sections is electrically isolated from the wall of the first tube and maintained at a DC voltage with respect to said wall. Ions of polarity are attracted toward the disk, while ions of opposite polarity are repelled from the disk. The other disk is maintained at a voltage with respect to the first disk to create an electric field in the electric field which forces selected ions of one polarity in a direction opposite to the flow of the gas sample. can. The walls of the second zone are connected to a first pole of a positive or negative DC power source for positively or negatively charged ions, respectively, and the walls of the first zone are connected to a second pole of a power source of opposite voltage to the first pole. It is convenient to connect to the poles. 1st
and the walls of the second zone can be maintained at a suitable voltage by a single power supply installed outside the hollow body and connected to these walls by electrical cables. Electrodes may be provided in the third zone, preferably aligned with the holes, to collect ions of a selected polarity. This electrode can be provided with an insulated lead that penetrates the wall of the hollow body, and can be electrically connected to a DC voltage source to attract ions of selected polarity, and means for measuring changes in current. can be connected to. Thus, in one example, the electrode is connected to the input terminal of a current amplifier and the output terminal of the current amplifier is connected to the indicating means to measure changes in current flow from the electrode. An amplifier can be arranged to have a relatively low gain when the current supplied to its input terminal is constant, and a significantly large gain with respect to changes in its input current. Feedback means is connected between the inverting input terminal of the amplifier and the tap point of the potentiometer bank, the potentiometer bank is connected between the output terminal of the amplifier and the common line, and the feedback means is connected between the tap point and the common line. Preferably, a capacitor is provided in the potentiometer array connected to the potentiometer array. The electrodes can be connected to the input terminals of the amplifier and the common line can be connected to a DC voltage source. The device of the invention may be provided with means for discharging the capacitor when the current collected by the electrodes decreases. This example can also be provided with means for comparing the voltage of the capacitor with the voltage at the output terminal of the amplifier and means for discharging the capacitor if the voltage on the capacitor is greater than the voltage at the output terminal of the amplifier. Such an arrangement may also be provided with means to prevent the capacitor from charging due to voltage surges in the power supply that may occur when the device is switched on. Switching means may also be provided which serves to connect the low impedance path between the capacitors when the device is switched on and to disconnect said low impedance path when the power supply to the device reaches a steady state. In another aspect of the present invention, an ion repelling electric field directed in the opposite direction to the gas flow is applied to the ions generated in the mixed gas sample under flow conditions, so that the oxygen-containing gaseous mixture is In detecting trace amounts of a chemical species in vapor form contained therein, the gaseous mixture is passed through the hollow body as described above, and a portion of the molecules of the species are ionized in a first region of the hollow body to form a certain polarity. ions of selected polarity are transferred forward with the sample of said gaseous mixture and passed with said gas stream at a substantially uniform flow rate into a second zone, into which selected ions of polarity are directed oppositely to said gas stream. forming an electric field which acts to push the gas in a direction through the third zone, without extending this electric field into the third zone, and which is then carried by the gas flow through the electric field in the second zone. A detection method for detecting ions having sufficiently low ion mobility is provided. This method can use atmospheric gas mixtures containing trace amounts of chemical species. Smoke-producing compositions can be detected, for example, by fumigating candles or powders that produce smoke of the compound upon combustion. This generated compound is combined with a combustible mixture, e.g. a fuel such as ground wood or sugar, and a substance to sustain combustion, e.g. ammonium nitrate or potassium chlorate, and/or a substance to retard combustion, e.g. kaolin, bentonite. and/or mixtures with silicic acid. Compositions containing trace amounts of chemical species can be detected from sprayed pesticides. It is known that when controlling harmful microorganisms in agriculture, horticulture or forestry, large areas to be treated can be sprayed with aqueous solutions or dispersions of pesticides. The amount of liquid used can vary within a wide range and can be applied from 40 to 1000/ha. These solutions can be applied by spraying from ground-based machines or aircraft, and are often applied from very small volumes of agricultural fluid. Such agents usually break up into a mist of very small droplets upon application, which droplets have a diameter of only about 100 μm and are still in vaporized form. The trace species described above can be easily detected by the method and apparatus of the present invention. The presence of an aerosol composition can also be detected, particularly when an aerosol composition containing a chemical species is prepared in a conventional manner, the active substance and a solvent and a volatile fluid such as a propellant, e.g. This is true when it contains a fluorine derivative. The invention will now be explained by way of example with reference to the drawings. First, as shown in FIG. 1, the detection device of the present invention comprises a generally cylindrical hollow body 1 with insulating coaxial tubes 2 and 3 of conductive material such as iron, copper or aluminum or alloys thereof. The hollow body 1 is formed by joining the two ends in an abutting relationship by means of a ring 4 of material. The tube 2 is provided at its free end with an end wall 5 of electrically conductive material, the end wall 5 being provided with a central hole 6. An inlet nozzle or probe 7 coaxial with the central bore 6 is provided on the outer surface of the end wall 5. The nozzle 7 can be made of an insulating material or of a conductive material, such as metal, in the latter case the nozzle 7 is surrounded by an insulating spacer ring. 8 from the end wall 5. A partition wall 9 is provided at the end of the pipe 2 adjacent to the pipe 3, and the partition wall 9
consists of a metal disc with an axial hole 10. A baffle plate 11 is installed midway between the ends of the tube 3 and is a sheet or lattice-like structure of metal disk or wire mesh having at least one hole 12 and electrically connected to the wall of the tube 3 around the baffle plate. Connect to. A fan 13 driven by an electric motor 14 is connected to a pipe 3.
installed adjacent to the opening. The fan 13 takes a sample of the atmosphere from the nozzle 7 and then sequentially samples the end wall and the first sample in the body 1 formed by the partition wall 9.
serving to discharge the sample from the open end of the tube 3 via a zone 15, a second zone 16 formed by the partition 9 and the baffle 11, and a third zone 17 formed by the baffle 11 and the fan 13. do. The apertures 10 in the septum 9 can be shaped so that the sample passes through the second zone 16 as a single smooth jet of substantially uniform velocity, as shown by the arrow 18. Hole 1
The appropriate shape for 0 is based on the British standard specification.
Standards Specification) No. 1042, page 141. In another arrangement, the septum 9 is provided with a plurality of holes, each hole being shaped so that the sample passes through the second zone 16 as a corresponding plurality of uniformly velocity smooth jets. To smooth the flow upstream of the partition wall 9, a grid 19 with a plurality of parallel passages is provided in the hole 6. A tapered electrode 20 is installed in the first section 15 and is supported by a lead wire 21 which is passed through an insulator 22 attached to the wall of the tube 2. The lead wire 21 is connected to one side of the high voltage power supply unit 23 via the resistor 71 on the outside of the tube 2, and the unit 23
Connect the other side to a common ground point. By means of the power supply unit 23, a corona discharge is generated at the electrode 20 and a high voltage, e.g. Generates 3000V. Other suitable high voltages used to generate the ionizing corona discharge may be between 2000 and 5000V. The resistor 71 has a value R selected such that the current I _D of the corona discharge is no less than 100 nA, preferably no less than 30 nA. The current _ID depends on the output voltage V ₃ of the feed unit 23, the value R of the resistor 71 and, to some extent, on the tapered electrode 2.
It depends on the shape of 0. This shape affects the ignition voltage of the discharge. This shape may change during use, for example due to erosion of the electrode, resulting in a change in the value of I _D. Another cause of the change in I _D is the feed unit 2.
It is 3. It may be advantageous for the power supply unit 23 to be an EHT generating unit energized by a low voltage source such as a 12V battery. This low voltage source can also be used to power other parts of the device as described below. Such EHT power generation units are well known and will not be described in detail. Since the state of charge of the accumulator varies with use, the output voltage V ₃ of such a unit may vary from its nominal value. For these reasons, for a particular nominal value of V ₃ and a predetermined value of R, the discharge current _ID may drop to a value within a certain range. Typical ranges of values of I _D for various values of R, where the nominal value of V ₃ is 3000V, are as follows: R=0 GΩ I _D =2 to 5 μA R=50 GΩ I _D =20 to 30 nA R=100 GΩ I _D =10 to 15 nA Therefore, the value of the resistor 71 is preferably 50 to 100 GΩ. Since the molecules of the species described above are only present at low concentrations in the sample, it is difficult to ionize the molecules of the species to an appreciable level by direct ionization. Ionizing the molecules of the species requires an extremely powerful electrical discharge, which, when achieved, produces extensive ionization of the air gas. This is avoided by ionizing a portion of the air gas such that the discharge allows a charge exchange reaction to occur, in which the charge of the air ions is transferred to the molecules of the species. Considering anions, the ionization of the molecules of the above chemical species present in low concentrations will occur if the molecules of the above chemical species have a greater electron affinity than any molecule present in relatively large numbers, i.e. air molecules. can be caused by such means. Electrons present in the gas quickly transfer to molecules with higher electron affinity. Moreover, if two chemical species with different electron affinities each exist in trace amounts, for example 1×10 ^-9 mol/mol, in a sample, both of them will have larger electron affinities than any arbitrary molecule present in large numbers. As long as the The reason for this is that molecules of chemical species with large electron affinities and
Collision with an ion of a species with a small electron affinity (in which case an electron is taken away from the ion)
However, because the concentrations of both chemical species are low, collisions between molecules of either chemical species and air ions (in which case molecules of either chemical species may be ionized) occur much less frequently. It is from. Similar considerations apply to cation populations. Since the ionic charge has a tendency to migrate to molecules of greater affinity, a significant proportion of the molecules of a particular species present in the air sample passing through the first zone 15 will be ionized. Below, a case will be described in which chemical species exist in the air at a concentration of, for example, 1×10 ⁻⁹ molecules/molecule.
For example, a corona discharge from electrode 20 can produce a primary (ie, air) ion density of 1×10 ^-8 ions/molecule to air in first zone 15 . Suppose that 1/10 of the existing chemical species is ionized by a charge exchange reaction. The concentration of species ions is therefore 1×10 ⁻¹⁰ ions/molecule. The primary ion concentration remains almost unchanged at 1×10 ^-8 ions/molecule. Therefore, the ions of the chemical species are 1×10 ⁻² for all ions present. Therefore, the detection problem changes from a problem of detecting 1×10 ⁻⁹ molecules to a problem of detecting 1×10 ^{−2 molecules} . That is, an improvement of 10 ⁷ times. The primary (air) ion population generated by the corona discharge in the first zone 15 is initially bipolar, but the strong electric field created in the vicinity of the electrode 20 due to the very high negative potential of the electrode 20 with respect to the tube 2 causes the positive ions to It moves towards the electrode and is neutralized by the electrode. Therefore, almost only the primary anions move with the air flow in the direction of the holes 10 in the partition wall 9. During the transition towards the primary anion barrier 9, a portion of the species molecules present in the air stream are ionized by the charge exchange reaction described above. Thus, the pores 10 are reached by a mixture of secondary (air) anions and secondary anions generated from species molecules. In the absence of an electric field, these ions are carried by the jet 18 across the second zone 16, past the baffle plate 11, and into the third zone 17. An electrode 24 is provided in this third area 17. The electrode 24 can be a wire, a grid, or a plate, and is connected by a lead wire 25. The lead wire 25 passes through an insulator 26 in the wall of the tube 3 to an input terminal 27 of a current amplification unit 28. The amplification unit 28 is provided with a common terminal, and this terminal is connected to the first
Since the storage battery 29 in the figure can be connected to a point with a relatively low positive potential, for example 5 V, with respect to the tube 3, the input terminal 27 and together with the electrode 24
is also held positive with respect to tube 3. Therefore the electrode 24
acts to attract anions flowing into the third zone. Such ions collected by electrode 24 cause a change in the input current of amplification unit 28. The resulting change in output current of the amplification unit may be measured by a meter 30 and/or an alarm circuit operative to utilize such change in output current to generate a signal, such as an audible or visible signal. 31 can be activated. For example, a voltage source of 300V indicated by a storage battery 32 is connected to the tube 2.
and the tube 3 to make the tube 3 and with it the partition wall 9 positive with respect to the tube 2 and in particular the baffle plate 11. An electric field is therefore created in the second area 19. Although a fringing effect occurs near the wall of the hollow body 1, the electric field near the jet 18 is approximately parallel to the axis of the jet and acts to push the anions in a direction opposite to the flow direction of the jet. The electric field strength near the jet 18 is determined by the distance between the partition wall 9 and the baffle plate 11 and by the voltage V of the voltage source 32. This value can be, for example, 3000V/cm. For a given electric field strength, ions whose ion mobility has a certain critical value are driven by the electric field exactly at the velocity of the jet, but in a direction opposite to the direction of the jet. The appropriate flow rate of the jet is 300
~400cm/sec. Such ions are stationary as far as their motion parallel to the jet axis is concerned. Such random lateral motion of the ions can move the ions laterally to areas of slow gas flow, and the electric field within such areas can therefore drive such ions back to the septum 9. Therefore, ions with high mobility are prevented from reaching the baffle plate 11 and from flowing into the third region 17 by the electric field. Ions with mobility greater than the critical value are
It is prevented from flowing into area 17. This is because such ions are driven in the opposite direction by the electric field in the second zone 16 at a faster flow rate than the jet. Ions with mobility smaller than the critical value are
The electric field in area 16 imparts a slower velocity.
Some of these ions may be transported by random lateral motion to areas where the velocity of the gas stream is slower than the velocity imparted to them by the electric field, but usually ions with ion mobilities below a critical value are Due to the jet, the second
It is transported across zone 16 and flows into third zone 17 where it is collected by electrode 24 . Due to the predetermined electric field strength and the predetermined geometry in the second zone 16, ions with a mobility equal to or greater than the critical value are
Doesn't have a chance of reaching 4. In the case of ions with a mobility less than a critical value, there is a constant probability that increases as the ion's mobility decreases and thus the time it takes for the ion to traverse the second zone 16 decreases. . In use, the value of the voltage V of the voltage source 32 is selected to eject substantially all primary (air) ions from the third zone 17, but remove even more primary (air) ions, such as those generated from certain types of chemical species. An electric field is created which causes the heavier and less mobile ions to flow into the third zone 17 and reach the collector electrode 24 . Such ions appear as an increase in the output current of amplifier 28, which can be read by meter 30 and used to activate alarm circuit 31. Passing a sample of air through the device produces a low, nearly constant electrical current (background current), which is indicated by meter 30. If desired, the meter can be mechanically or electrically offset to provide a zero reading under such conditions. When a sample of air containing trace amounts of a particular species is passed through the device, the meter reading increases by a value determined by the concentration of the species. The alarm circuit 31 may be arranged to issue an alarm signal if the output current of the amplification unit 28 exceeds a limit level which is greater than the background current level. The apparatus described with respect to FIG. 1 is suitable for detecting the presence of chemical species that form anions by charge exchange reactions. Cations that may form within the first zone 15 are attracted to the discharge electrode 20;
Do not enter the second area 16. To make it suitable for detecting chemical species forming cations, the high voltage power supply unit 23 and voltage source 29 of the device are
It is only necessary to reverse the polarity of and 32. The amplifier unit 28 is of a type with high DC stability (low drift) and relatively high gain, so that small changes in the input current, such as may be caused by the presence of very low concentrations of certain species, It is preferable to be able to make large changes in the output current. Changes in the amplifier's output current are masked by background current variations caused by drift in the amplifier unit. FIG. 2 shows the basic arrangement, and FIG. 3 shows a detailed circuit diagram of the current amplifier used as the amplification unit 28. First, as shown in Figure 2, input terminal 2
7 is connected to the inverting input terminal 33 of the operational amplifier 34, and its non-inverting input terminal 35 is connected to the common line 36. Resistors 37 and 38 and capacitor 39 are connected in series between operational amplifier 34 and common line 36. The feedback resistor 41 is connected to the resistors 37 and 3.
8 and the inverting input terminal 33. For example, Burr Brown (product name, Burr)
Brown) amplifier type 3527AM can be used as amplifier 34. The resistance values of resistors 37, 38 and 41 are 33MΩ, 330KΩ and 330KΩ, respectively.
The capacitance of the capacitor 39 can be set to 5000 MΩ, and the capacitance of the capacitor 39 can be set to 47 μF. Since the capacitor 39 is effectively open circuit at zero frequency, the DC gain of this arrangement is 1, and therefore the DC stability is high (low drift).
I understand that. At frequencies where the impedance of capacitor 39 is less than the impedance of resistor 38, the gain is determined by the ratio of resistors 37 and 38. For the values exemplified above, it can be seen that the gain of this arrangement is 100 when the frequency is reduced to a value as low as 1 Hz. There are certain problems when using the arrangement of FIG. 2 for amplification unit 28. When passing an atmospheric sample containing a particular type of chemical species through the device, the electrode 24
A negative-going current pulse is supplied to the input terminal 27 by the generated anions reaching . At the rising edge of this current pulse, the output terminal 40 points positive;
Capacitor 39 is charged. At the end of this current pulse, output terminal 40 does not immediately return to its quiescent value (background current), but is held positive by the charge on capacitor 39. Therefore, in this example:
The device would not be able to respond to the second sample until the charge on capacitor 39 created by the first sample has leaked away. Using the component values listed above and capacitor 39
It can be seen that ignoring the internal leakage in the system, it may take several hours for the device to recover. A similar effect can occur when the device is first switched on, if the initial surge current can leave the capacitor 39 with a positive charge, and this charge is removed before the device is ready for operation. You have to wait until it leaks and disappears. The arrangement of FIG. 3 provides a means to overcome both such drawbacks. In FIG. 3, an operational amplifier 34, a resistor 3
7, 38 and 41 and capacitor 39 to the second
Connect as described for the diagram. The common line 36 is connected to a negative bus 42 from a DC power source via a Zener diode 43. The positive supply terminal of amplifier 34 is connected from the DC power supply to positive bus bar 44 via switch 45 . Therefore, the common line 36
And with this, the input terminal 33 of the amplifier 34 is held positive with respect to the negative bus 42 by the zener voltage V ₁ of the diode 43 (for example 5V). Bus line 42
is grounded and connected to pipe 3 (Fig. 1). The electrode 24 is therefore held at a positive voltage V ₁ with respect to the tube 1, and the Zener diode 43 is connected to the voltage source 29 of FIG.
Configure. The other amplifier 46 is provided with an inverting input terminal 47,
This terminal is connected to the output terminal 40 of the amplifier 34. The non-inverting input terminal 48 of the amplifier 46 is connected to the resistor 49.
Connection point 5 between resistor 38 and capacitor 39 via
Connect to 0. A diode 51 is connected between the output terminal 52 of the amplifier 46 and its non-inverting input terminal 48. This is an arrangement such that when input terminal 47 is positive with respect to input terminal 48, diode 51 does not conduct. Input terminal 48 is input terminal 4
7, i.e. capacitor 3
9 is positively charged with respect to the output terminal 40 of the amplifier 34, the diode 51 is conductive and the capacitor 39 is discharged through the resistor 49. The resistance value of the resistor 49 is, for example, 33KΩ. Capacitor 39 is charged when a negative going current pulse is applied to input terminal 27, but is quickly discharged after the pulse is gone so the device can respond to the next sample. When switch 45 is closed for the first time, capacitor 3
9 is not charged, a pnp transistor 53 is provided, the collector of which is connected to the connection point 50. The base of the transistor 53 is connected to the connection point 55 between the resistor 56 and the capacitor 57 via the resistor 54, and the resistor 56 and the capacitor 5
7 is connected in series between the positive bus bar 44 and the negative bus bar 42. A resistor 58 is connected between the positive bus 44 and the emitter of transistor 53. The anode of diode 59 is connected to the emitter of transistor 53 and its cathode is connected to common line 36. When the switch 45 is closed, the node 55 and with it the base of the transistor 53 are initially at the voltage of the negative bus 42. Therefore transistor 5
3 conducts and clamps the node 50 to the voltage of the common line 36. The voltage at node 55 rises as capacitor 57 charges through resistor 56, possibly cutting off transistor 53 and thus unclamping node 50. The values of resistor 56 and capacitor 57 are such that transistor 53 remains conductive at least until amplifier 34 reaches its steady state and then closes switch 45, thus preventing charge from building up in the capacitor due to closing the switch. be selected. To facilitate steady state setting of the amplification unit (background current), a potentiometer 60 can be provided. One end of this potentiometer can be connected to the positive bus bar through a resistor 58, and the other end can be connected to the common line 36 through a resistor 61, the resistors 58 and 61 being able to adjust the position of the slide 62. This acts to limit voltage changes that may occur due to Slider 62, for example
Feedback resistor 41 and resistor 3 via 33MΩ resistor 63
Connect to the connection point with 7 and 38. By adjusting the potentiometer, the background current level can be set to the desired value. The negative bus bar 42 and the positive bus bar 44 can be connected to corresponding terminals of a storage battery 70 with an output of 12V, for example. Battery 70 can also be used to energize motor 14 and voltage sources 23 and 32 via on-off switch 45 (FIG. 1). Voltage source 23 provides the high voltage necessary to create a corona discharge within first zone 15. Voltage source 23 advantageously constitutes a well-known EHT generation circuit. Similarly, voltage source 32 may constitute an EHT generator or a DC-DC converter of known types. Studies have shown that the effect of external airflow is significantly reduced when the sample is discharged into the atmosphere in an area adjacent to the inlet nozzle, subjecting the sample to approximately similar external conditions as the inlet nozzle. For this purpose, the hollow body 1 is surrounded by an outer casing 64. The casing 64 has an end wall 6 spaced from the open outlet end of the tube 3.
5, providing a passage for the air flow discharged by the fan 13 to reach the atmosphere via an annular space 66 between the hollow body 1 and the outer casing 64, and then through an annular hole 67 surrounding the inlet end of the hollow body 1. provide. The exhaust air passages are indicated by arrows 68 and 69. The accumulator 70, the voltage sources 23 and 32, the amplifier unit 28 and the alarm device 31 can be installed in the outer casing to obtain a hand-held device. In another arrangement, the primary ionization is carried out by a source of ionized radiation (22' in FIG. 4) placed in a container 20' (FIG. 4) in the first zone, for example alpha particles such as americium-241. emitters, or β-particle emitters such as Nickel 63 or tritium. Since the primary ion population generated in this way is bipolar, an electric field is applied to select ions of a certain polarity so as to move them forward with the airflow, and to prevent ions of the opposite polarity from approaching the partition wall 9. must be formed in the first area 15. One way of achieving this is to replace the bulkhead 9 (FIG. 4) with a ring 4' of insulating material.
The partition wall 9 is insulated from the tube 2 by applying a suitable voltage between the partition wall 9 and the tube 2. Further, in FIG. 1, when primary ionization is performed by a source of ionized radiation installed in a container (not shown) in the first zone 15, the partition wall 9 is insulated from the tube 2. The electrode 20, lead wire 21, insulator 22 and high voltage power supply unit 23 can be omitted. A container 20' containing a source of radiation in an ionizing state can be inserted into the first section 15 through a hole (FIG. 1) provided for the insulator 22 in the wall of the tube 2. A similar arrangement can be used in the device of FIG. 4, in which holes (not shown) are provided in the walls of tube 2 and casing 64. This arrangement alleviates the problem of preventing leakage of atomic radiation from the device. Fields of application of the invention include counter-terrorist measures, such as the examination of travelers and luggage for explosives at airway terminals and similar locations, and the search for explosives in general. The device of the invention can also be used as a detector in gas chromatography. Another field of application is the search for leaks in equipment such as piping, pressure vessels, chemical plants, etc., where tracer substances such as heavy halogens are introduced into the equipment under test,
The device of the invention is moved throughout the exterior of the device to detect any escape of tracer material. In a representative experiment using an example apparatus such as that described in connection with FIG.
/ minute. The voltage V ₃ of the corona discharge electrode 20 is
3000V, the voltage V ₁ of the collector electrode 24 is 5V, and the strength of the repulsive electric field in the second region 16 is
It was set to 300V/cm. Resistor 71 was shorted so that I _D was 2 to 5 μA. The current flow collected by electrode 24 (ie, background current) was 1 pA when standard pressure air was flowed through the device. mixture of nitroglycerin vapor mixed with air
When a mixture having a known concentration of 1 part by weight of nitroglycerin to ¹⁰⁹ parts by weight was passed through the device, the current at the collector electrode 24 was 5 pA. The 4 pA increase was caused by the presence of nitroglycerin vapor in the sample. The experiment was then repeated using a resistor 71 having a value of GΩ in the circuit so that the discharge current did not exceed 30 nA. At this time, the background current was 0.3 pA, and the current when the nitrogerin sample was passed through the device was 1.0 pA. The reduction in background current is due to less noise produced by the limited current corona discharge. In another series of experiments, samples of various obtruding substances and samples of various explosives were firstly present in a device with a resistor 71 shorted, and secondly with a value of 50 GΩ. A resistor was present in the device incorporated into the circuit. All other parameters of the apparatus were kept constant throughout the experiment. The results obtained are shown in the following table. The table shows meter 30 readings for each substance. 0 to 100 (=full scale deflection) on the instrument
It had an optional direct scale.

[Table] Sieve ^*

[Table] * indicates the product name.

[Brief explanation of the drawing]

Fig. 1 is a sectional view of one example of the device of the present invention, Fig. 2 is a partial circuit diagram used in the device of the present invention, and Fig. 3 is a detailed circuit diagram of an amplification unit used in the device of Fig. 1. , FIG. 4 is a sectional view of another example of the device of the present invention. 1...Hollow body, 2, 3...Pipe, 4, 4'...Ring of insulating material, 5...End wall, 6...Central hole, 7...
Inlet nozzle (probe), 8... Insulating spacer ring, 9... Partition wall, 10... Hole, 11... Deflector plate, 12... Hole, 13... Fan, 14... Electric motor, 15... First interior Area, 16... Second internal area, 17... Third internal area, 18... Arrow indicating the direction in which the sample passes as a jet (jet stream), 19...
Grid, 20...electrode, 20'...container, 21...
Lead wire, 22... Insulator, 22'... Source of radiation in an ionized state, 23... High voltage power supply unit (voltage source), 24... Electrode, 25... Lead wire, 2
6...Insulator, 27...Input terminal, 28...Amplification unit (amplifier), 29...Storage battery, 30...
...Instrument, 31...Alarm circuit, 32...Storage battery (voltage source), 35...Non-inverting input terminal, 36...Common line, 37, 38, 41...Low resistance, 39...Capacitor, 40... Output terminal, 42... Negative bus bar, 43... Zener diode, 44... Positive bus bar, 45... Switch, 46... Amplifier, 47...
...Inverting input terminal, 48...Non-inverting input terminal, 49
... Resistor, 51 ... Diode, 52 ... Output terminal, 53 ... Transistor, 54, 56, 58,
61... Resistor, 57... Capacitor, 59... Diode, 60... Potentiometer, 62...
Slider, 64... Outer casing, 65... End wall, 66... Annular space, 67... Annular hole, 68,
69...Arrow indicating exhaust air passage, 70...Storage battery, 71...Resistor, 33...Inverting input terminal, 34
……amplifier.

Claims (1)

[Claims] 1. By applying an ion repelling electric field directed in the direction opposite to the gas flow to ions generated in a mixed gas sample under flow conditions, ions contained in an oxygen-containing gaseous mixture can be produced. In an apparatus for detecting trace amounts of chemical species in vapor form, means are provided for passing said sample through a hollow body, said hollow body having in series an inlet, a first internal zone, a second internal zone, a third internal zone and an outlet. providing, in the first zone, means for ionizing at least a portion of the molecules of the sample containing molecules of the chemical species, and means for selecting ions of a certain polarity for migration with the sample to the second zone; Further, means for promoting the flow of the sample through the second zone as a jet with a substantially uniform flow velocity, means for generating an ion repelling electric field in an opposite direction in the second zone, and collecting ions of a selected polarity in a third zone. and means for preventing, during operation, ions of a selected polarity having an ion mobility greater than a certain value depending on the strength of the ion repelling electric field and the flow rate of the gas stream from entering the third zone. A device for detecting trace amounts of chemical species in the form of vapor, characterized in that the device is arranged in such a way that it can detect trace amounts of chemical species in the form of vapor. 2. A hollow body is constituted by a first tube and a second tube of conductive material joined in a butt-to-end relationship by a ring of insulating material, with said first section disposed within the first tube, and said second section and said second tube disposed within said first tube. 2. The device of claim 1, wherein the three zones are located within the second tube. 3. The first tube is provided with an end wall having an inlet hole for introducing the sample into the first zone at the end opposite to the second tube, and a disk of conductive material at the end adjacent to the second tube. ,
3. Apparatus as claimed in claim 2, in which the disk is placed transversely to the gas flow and further includes at least one hole in the disk for introducing the sample into the second zone. 4 A hole provided in the disk at the end adjacent to the second pipe,
The sample flows in the second phase like a laminar flow with a nearly constant flow velocity.
4. The device of claim 3, shaped to pass through the area. 5. The second zone and the third zone are partially separated by another disk of conductive material placed at a point far from both ends of the second tube, and the disk is used to introduce the sample into the third zone. Claims 2 to 4 further include at least one hole communicating from the second section to the third section for the purpose of electrically contacting the disk with the tube wall.
Apparatus as described in any one of the headings. 6. The means for ionizing the sample molecules containing the molecules of the chemical species is an electrode installed in the first zone, and in use the means is electrically connected to a high-voltage DC power source to produce a corona discharge in the first zone. Claim 1, which produces an electric field that selects ions of a certain polarity for causing the ions to move along with the sample into the second zone.
5. The device according to any one of items 5 to 5. 7. The device according to claim 6, which limits the corona discharge current to 100 na or less. 8. The means for ionizing the sample molecules containing molecules of the chemical species is a radioactive ionization source installed in the first zone, and the disk between the first zone and the second zone is electrically insulated from the wall of the first tube. and maintaining a DC voltage with respect to the wall such that ions of one polarity are attracted towards the disk and ions of the opposite polarity are attracted towards the wall. 5. The device according to any one of items 5 to 5. 9 Another disk is maintained at a certain voltage with respect to the first disk to generate an electric field that exerts a force on selected ions of a certain polarity in a direction opposite to the flow of the sample. 9. The device according to any one of items 1 to 8. 10 The means for collecting ions of a selected polarity is an electrode provided in the third region of the hollow body, this electrode is electrically connected to the input terminal of a current amplifier, and this output terminal is connected to a DC voltage source. Claim 1, which attracts ions of polarity selected by
9. The device according to any one of items 9 to 9. 11 The amplifier is provided with feedback means connected between the inverting input terminal of the amplifier and the tap point of the potentiometer bank, the potentiometer bank being connected between the output terminal of the amplifier and the common line, and further connected between the tap point and the tap point. A capacitor connected to a common line is placed in the potentiometer string so that the gain of the amplifier is low for approximately constant input currents and relatively high for varying input currents. The device according to claim 10. 12. Claim 1 comprising means for comparing the voltage of the capacitor and the voltage at the output terminal of the amplifier, and means for discharging the capacitor when the voltage on the capacitor is greater than the voltage at the output terminal of the amplifier.
The device according to item 1. 13. Claim No. 1, comprising switch means for connecting a low impedance path between the capacitors when the device is switched on and for disconnecting said low impedance path when the power supply to the device reaches a steady state. The device according to item 11.